Typically, rotary wing aircrafts like helicopters are sustained by a rotor, spinning about a vertical rotor shaft, generating lift or upward thrust. In a conventional helicopter the thrust from the rotor can be controlled by changing the pitch angle 10 (or in short; the blade pitch) of the rotor blades. The blade pitch is in the field of propeller aerodynamics defined as the lateral angle between the blades and a reference plane perpendicular to the rotor shaft axis, measured perpendicular to the longitudinal axis of a rotor blade.
By collectively changing the blade pitch of all the rotor blades or by changing the angular velocity of the rotor, the helicopter can be controlled in the vertical direction. The horizontal direction of flight and the stability of the helicopter, however, are controlled by cyclically adjusting the blade pitch of individual blades. Cyclically adjusting the pitch means that the blade pitch of each rotor blade is adjusted from a maximum in a particular position of rotation to a minimum at the opposite side. This causes the lift in one part of the rotation to be larger than in other parts, whereby the rotor is tilted with respect to the reference plane. When the rotor (and helicopter) tilts like this, the initially vertical thrust also tilts, and therefore gets a horizontal component pulling the helicopter in the desired direction.
Normally, a helicopter must be actively controlled by a well trained pilot or from gyroscopic sensors and computers. The necessary means to varying and controlling the pitch angle of each blade are normally complicated, expensive and add weight to the helicopter. The blade pitch is typically controlled via a swash plate connected to servos. Because the swash plate needs to be positioned accurately with as little friction and play as possible it is complicated and expensive. On most helicopters the swash plate has a spinning part and a non-spinning part connected together with a large ball bearing. The spinning part of the swash plate is again connected to the rotor blades via a set of links and other mechanical components.
Alternative solutions employing actuators connected to rotor blade control surfaces or magnetic coil systems acting directly on a permanent magnet mounted on a rotor blade pitch arm have been tested. Control of rotary wing aircrafts has traditionally been challenging.
The rapidly spinning rotor behaves as a gyroscope and introduces precession. When a torque is applied perpendicular to the axis of rotation of a gyroscope, the resulting motion is perpendicular to both the axis of spinning and the applied force. The angular velocity of the motion is proportional to 20 the applied torque.
Under normal flying, the fuselage does behave differently, and does not introduce precession. When a torque is applied to the fuselage, the result is an angular acceleration, parallel with and proportional to the applied torque.
Controlling a rapidly spinning gyroscope alone is straightforward. The applied torque must be applied 90 degrees in advance of the required motion, and must be proportional to the required angular velocity. I.e. for a required roll motion, a pitch torque must be applied. For a pitch motion, a roll torque must be applied. The torque must be maintained for the complete duration of the motion. The angular motion will stop if the torque vanishes.
Control of a fuselage alone is similarly straightforward. The applied torque must be applied parallel to the required motion, and must be proportional to the required angular acceleration. The torque must be applied only during the acceleration and retardation phase of the motion, once an angular velocity is achieved, this velocity will remain constant if without any applied torque.
The challenge arises when the rotor is fixed to the fuselage. When fixed together, with exception of the one degree of freedom of the connecting joint, both parts must rotate in parallel, and when a torque is applied, the resulting motion arising from the torque will be somewhat between parallel (fuselage) and 90 degrees behind (rotor) the applied torque. The exact angle is dependent on the current state of the aircraft.
Prior art usually consider this angle to be constant for a rotary wing aircraft. For some aircrafts, the angle can be tuned to a working compromise. Often, 45 degrees is chosen, as this seems to give the most robust solution. Regardless of the 20 angle, any constant value will result in a heavily coupled system. Any applied torque will result in a motion both parallel as well as a motion perpendicular to the torque. When designing automatic control systems, this cross coupling will cause multiple feedback paths, making it more difficult to achieve necessary control authority and necessary stability margins. Even when stable, any motion change, regardless of being a requested motion or a correction of an undesired motion, will be achieved through multiple passes through the regulator, each correcting the error motion from the last pass. The aircraft will obtain its requested attitude (meaning roll and pitch) in an oscillating approach.
For some aircrafts, typically smaller aircraft with fast dynamics, or aircrafts with especially strict demands on control, the traditional approach does not work.